Several building materials are designed with sound attenuation or absorption properties in mind as it is often desirable to minimize, or at least reduce, the amount of sound that can be heard across a partition. With respect to building structures, building materials such as wallboard, insulation, and often paint, are considered materials that can contribute to a reduction in sound.
Wallboard is a common utility or building material, which comes in many different types, designs, and sizes. Wallboard can be configured to exhibit many different properties or characteristics, such as different sound absorption, heat transfer and/or fire resistance properties. By far, the most common type of wallboard is drywall or gypsum board. Drywall comprises an inner core of gypsum, the semi-hydrous form of calcium sulphate (CaSO4.½H2O), disposed between two facing membranes, typically paper or fiberglass mats. Drywall may comprise various additives and fillers to vary its properties.
The most commonly used drywall is one-half-inch thick but can range from one quarter (6.35 mm) to one inch (25 mm) in thickness. For soundproofing or fire resistance, two layers of drywall are sometimes laid at right angles to one another. Drywall provides a thermal resistance, or R value, of 0.32 for three-eighths-inch board, 0.45 for half inch, 0.56 for five-eighths inch and 0.83 for one-inch board. In addition to increased R-value, thicker drywall has a slightly higher Sound Transmission Class (STC) rating.
STC, part of ASTM International Classification E413 and E90, is a widely used standard for rating how well a building material attenuates airborne sound. The STC number is derived from sound attenuation values tested at sixteen standard frequencies from 125 Hz to 4000 Hz. These transmission-loss values are then plotted on a sound pressure level graph and the resulting curve is compared to a standard reference contour. Acoustical engineers fit these values to the appropriate TL Curve (or Transmission Loss) to determine an STC rating. STC can be thought of as the decibel reduction in noise that a wall or other partition can provide. The dB scale is logarithmic, with the human ear perceiving a 10 dB reduction in sound as roughly halving the volume. Therefore, any reduction in dB is significant. The reduction in dB for the same material depends upon the frequency of the sound transmission. The higher the STC rating, the more effective the barrier is at reducing the transmission of most common sound frequencies.
Conventional interior walls in homes or buildings have opposing sheets of drywall mounted on a stud frame or stud wall. In this arrangement, with the drywall panels having a ½ inch thickness, the interior wall measures an STC of about 33. Adding fiberglass insulation helps, but only increases the STC to 36-39, depending upon the type and quality of insulation, as well as stud and screw spacing. As wallboard is typically comprised of several sheets or panels, the small cracks or gaps between panels, or any other cracks or gaps in the wall structure are referred to as “flanking paths,” and will allow sound to transmit more freely, thus resulting in a lower overall STC rating. For this reason it is critical that all potential flanking paths be eliminated or reduced as much as possible.
Similarly, the Outdoor-Indoor Transmission Class (OITC) is the widely used standard for indicating the rate of transmission of sound between outdoor and indoor spaces. OITC testing typically considers frequencies down to 80 Hz and is weighted more to lower frequencies.